Deformations present within a sheet of a print medium in a printing system can be problematic for various reasons. Firstly, one or more such deformations can cause serious reliability problems in a printing system, such as an inkjet printing system, where there is only a small gap between a sheet transport mechanism and an image forming device or printing head of the printing system. If the sheet to be printed touches the image forming device or the printing head as a result of such a deformation, this can lead to print quality degradation and/or to a sheet jam in the machine. To achieve high print quality in an inkjet printing system, the distance between the printing heads and sheet to be printed should be kept small. Because of this small distance (print gap) the print heads are easily touched by the sheets as they pass. Accordingly, even small defects like dog ears, wrinkles, tears etc. can cause a so-called “head touch”, which can degrade print quality, cause nozzle failure, or even sheet jams. Secondly, if the sheets of printed medium output from the printing system include any such deformations, this naturally compromises the quality of the output. Depending on the degree or extent of the deformations in the printed sheets, those sheets may need to be discarded and re-printed.
To address these issues, systems have been developed which employ a proofing device capable of identifying sheet deformations and rejecting sheets that contain such deformations. However, there are many sources of defects or errors that may degrade the productivity of a printing system. For example, sheets to be printed supplied to a printing machine may already contain various defects. Also, defects and wear within the machine can cause the sheets to become damaged. Changes in the environmental conditions can lead to deformation of the sheets as they are being processed, and inappropriate settings in the printing system, such as too much ink or a drying temperature that is too high, can also generate problems. In addition, a transport mechanism in the printing system will typically employ an under-pressure or suction for holding sheets of the print medium. If the under-pressure or suction is insufficient, deformations or wrinkles known as “cockling” can occur in the sheets, particularly during drying and/or fixing of an image after a printing operation. These influences or defects may also act in combination, thus making it very difficult to identify a root cause of a problem.